Fusion welded joints formed from parent metals of aluminum alloys containing zinc and magnesium are susceptible to corrosion, particularly at regions of the weld where a magnesium-rich segregate has formed. Three types of corrosion can occur, namely exfoliation corrosion, weld toe corrosion and stress corrosion, all of which can seriously degrade the mechanical strength of the weld.
These corrosion types are herein defined with reference to the accompanying drawings.
The T-joint illustrated in FIG. 1 comprises an edge plate 1 and a face plate 2 welded together by two fillet welds 3 formed by a conventional metal-inert-gas (MIG) process. Each weld has a fusion zone 4, a heat affected zone 5, a weld zone 6 and a weld toe 7.
Exfoliation corrosion manifests itself as a groove 8 in the heat affected zone 5 parallel with the line of the weld.
Weld toe corrosion occurs at the weld toe 7, where segregate concentration can be high, and once initiated can lead to a crack 9 which extends into the plate 2 along the interface of the fusion zone 4 and the heat affected zone 5. This effect is sometimes known as `white zone cracking` or `boundary bead cracking`.
An effect of stress corrosion, sometimes known as "hidden edge cracking", is illustrated in FIG. 2. The face plate 2 of this L-joint is provided with a rebate 10 within which the edge plate 1 is located. Remanent moisture trapped within the rebate 10 can lead to corrosion at the interfaces of the plates 1 and 2 which manifests itself as a crack or cracks in the parent metal, extending from the interfaces in a plane parallel with the respective plate, eg cracks 11 and 12 in the plates 1 and 2 respectively.
It is known to minimize the effect of the second of these corrosion types, ie weld toe corrosion, by removing the segregate from the weld toes immediately after welding, either by mechanical means or with a laser beam.
Attempts have also been made to prevent deposition of the segregate at the weld toe during the welding process, for example, by the application of a repellant coating to the parent metal adjacent to the weld toe location prior to welding so as to cause the segregate to flow outwardly from the weld toe during the welding process. Subsequent weld toe cracking can be successfully eliminated in this way but the lateral displacement of the segregate thus caused can exacerbate the exfoliation corrosion problem.
It is also well known to provide the exterior surfaces of steel structures with protection against environmental corrosion by application of a coating of a galvanically protective metal, and the present invention seeks to employ the principle of galvanic protection in a manner which can combat all three of the aforesaid corrosion problems.